Image forming apparatus

Abstract

An image forming apparatus includes a photosensitive member, a developing member to supply a toner to the photosensitive member at a developing portion, and a transfer member to transfer a toner image to a recording material passing through a transfer portion from the photosensitive member. In a case which the transfer is performed to a shorter width recording material and thereafter, the transfer is successively performed to a longer width recording material, a control portion controls a first back contrast formed when an area of a surface of the photosensitive member corresponding to the transfer portion through which the shorter width recording material is passing reaches the developing portion, and a second back contrast formed when an area of the surface corresponding to the transfer portion through which the longer width recording material is passing reaches the developing portion so that the second back contrast is higher than the first back contrast.

What is claimed is: 
     
       1. An image forming apparatus comprising:
 a rotatable photosensitive member; 
 a charging roller configured to perform a charging process of a surface of the photosensitive member; 
 a charging voltage power source configured to apply a charging voltage for the charging process to the charging roller; 
 an exposure portion configured to expose the charging processed surface of the photosensitive member and to form an electrostatic image on the photosensitive member; 
 a developing member configured to supply toner to the electrostatic image on the photosensitive member at a developing portion and to form a toner image on the photosensitive member; 
 a developing voltage power source configured to apply a developing voltage for the developing to the developing member; 
 a transfer member configured to form a transfer portion by contacting the photosensitive member and to transfer the toner image to a recording material passing through the transfer portion from the photosensitive member; 
 a transfer voltage power source configured to apply a transfer voltage for the transfer to the transfer member; and 
 a control portion configured to control at least one of the charging voltage power source, the developing voltage power source and the exposure portion, 
 wherein an absolute value of a difference between a potential of a non-image portion of the surface of the photosensitive member at the developing portion and a potential of the developing voltage is defined as a back contrast, and 
 wherein, in a case which the transfer is performed to a first recording material of which width in a direction substantially perpendicular to a feeding direction of the recording material is a first width, and thereafter, the transfer is successively performed to a second recording material of which width is a second width longer than the first width, 
 the control portion controls a first back contrast which is the back contrast to be formed when an area of the surface of the photosensitive member corresponding to the transfer portion through which the first recording material is passing reaches the developing portion, and a second back contrast which is the back contrast to be formed when an area of the surface of the photosensitive member corresponding to the transfer portion through which the second recording material is passing reaches the developing portion so that the second back contrast is higher than the first back contrast. 
 
     
     
       2. The image forming apparatus according to  claim 1 , wherein the control portion controls the charging voltage power source to change a surface potential of the photosensitive member formed by the charging process of the charging roller so that the second back contrast is higher than the first back contrast. 
     
     
       3. The image forming apparatus according to  claim 1 , wherein the exposure portion exposes the charging processed surface of the photosensitive member with a first output for forming the potential of the non-image portion and a second output for forming a potential of an image portion, and forms the electrostatic image on the photosensitive member, and
 wherein the control portion controls the exposure portion to change the first output so that, of the second back contrast, only the second back contrast in the area of the surface of the photosensitive member which does not contact the first recording material at the transfer portion and contacts the second recording material at the transfer portion is higher than the first back contrast. 
 
     
     
       4. The image forming apparatus according to  claim 1 , wherein the control portion controls to change a value of the second back contrast higher than the first back contrast according to the first width. 
     
     
       5. The image forming apparatus according to  claim 4 , wherein the control portion controls so that the value of the second back contrast higher than the first back contrast becomes higher in a case in which the first width is a second value greater than a first value than in a case in which the first width is the first value. 
     
     
       6. The image forming apparatus according to  claim 1 , wherein the control portion controls to change a value of the second back contrast higher than the first back contrast according to a number of the first recording material of which width is the first width and to which the transfer is successively performed before the transfer is performed to the second recording material. 
     
     
       7. The image forming apparatus according to  claim 6 , wherein the control portion controls so that the value of the second back contrast higher than the first back contrast becomes higher in a case in which the number is a second number greater than a first number than in a case in which the number is the first number. 
     
     
       8. The image forming apparatus according to  claim 1 , wherein, in a single print job which is a series of operations which is started by a single start instruction and in which the transfer is successively performed to a plurality of the recording material, the control portion controls so that the second back contrast is higher than the first back contrast in a case that the transfer is successively performed to the first recording material and the second recording material. 
     
     
       9. The image forming apparatus according to  claim 1 , wherein the control portion controls so that the second back contrast is higher than the first back contrast in a case that the transfer is successively performed to the first recording material and the second recording material without stopping rotation of the photosensitive member. 
     
     
       10. The image forming apparatus according to  claim 1 , wherein, in a case which the transfer is performed to the second recording material, and thereafter, the transfer is successively performed to a third recording material of which width is the second width, the control portion controls so that a third back contrast which is the back contrast to be formed when an area of the surface of the photosensitive member corresponding to the transfer portion through which the third recording material is passing reaches the developing portion is higher than the first back contrast.